decompr: Global-Value-Chain Decomposition

Two global-value-chain decompositions are implemented. Firstly, the
Wang-Wei-Zhu (Wang, Wei, and Zhu, 2013) algorithm splits bilateral gross exports
into 16 value-added components. Secondly, the Leontief decomposition (default)
derives the value added origin of exports by country and industry, which is also
based on Wang, Wei, and Zhu (Wang, Z., S.-J. Wei, and K. Zhu. 2013. "Quantifying
International Production Sharing at the Bilateral and Sector Levels.").